Pressure relief valves electrically adjusted by a proportional magnet are widely known in the art. The spring urging the closing element of the valve on the valve seat is biased by the force of the proportional magnet. Thereagainst the system pressure acts on the closing element in a sense to lift it off the valve seat. Since the maximum magnet force produced by a proportional magnet is limited, the diameter of the valve seat must be chosen to be relatively small when high system pressures have to be controlled. For example, a magnet having a cross section of 60 times 60 mm exercises a maximum force of 16 kp. Up to a pressure of 315 bar to be controlled, the magnet allows a diameter of 2.3 mm for the valve seat.
Furthermore, the size of the valve is relatively large due to the large dimensions for the magnet. Moreover, the flow cross section between the port leading to the system pressure and the return port to the reservoir is very small.